In a bicycle transmission system, the combination of a guide toothed wheel with a small diameter with a sprocket with a large diameter allows demanding climbs to be easily handled. However, this same combination, over flat land or going downhill, is disadvantageous since the cyclist's energy is dissipated, due to the fact that the cyclist is obliged to pedal quickly while the bicycle moves forwards slowly.
In order to make the aforementioned combination more suitable for the route to be made, it is known to equip the bicycle with a plurality of guide toothed wheels and with a plurality of sprockets, which can be combined with each other based upon requirements, through appropriate gearshifting devices.
Because bicycle components have reached a high degree of standardization, the characteristic size of components such as frame, gearshifting devices, freewheel body or chain do not greatly vary, so that most bicycles use three guide toothed wheels and up to ten sprockets.
However, certain bicycle transmission systems show a sprocket assembly comprising more sprockets, the largest of which are arranged partially over the spokes of the wheel of the bicycle. The sprockets have a minimum thickness, in the area of engagement with the chain, equal to 1.76 mm, whereas the distance between adjacent sprockets is lower than the minimum thickness of the sprockets themselves, and in particular is equal to 1.66 mm. In order to be made to rotate, the sprocket assembly illustrated requires the use of a special-shaped chain. The aforementioned chain comprises, in alternating succession, a first link, consisting of two plates that are parallel and spaced apart to define the area of engagement with a tooth of a sprocket, and a second link, consisting of a single plate shaped like a “bridge” so as to define in a central portion thereof a seat for coupling with the next tooth of the sprocket.
The aforementioned chain, thanks to the second link made with a single plate, has a maximum thickness much smaller than a conventional chain and for this reason it can also be used when there is a space between the sprockets that is smaller than the thickness of the sprockets themselves.
The aforementioned chain wears down quickly because of the continuous friction between the teeth of the sprockets and the second links consisting of a single shaped plate. This makes it subject to premature structural yielding.
Further, in such an arrangement, the sprockets that are arranged over the spokes of the wheel have a large diameter, i.e. they have a very high number of teeth. Since the number of teeth of the smallest sprocket of known sprocket assemblies is now standardized and goes up to about 11-12 teeth, it follows that passing from the smallest sprockets to the largest ones there is a non-linear increase in diameters, and therefore in the number of teeth. In other words, the cyclist notices a great difference in effort necessary to pedaling from one sprocket to an adjacent sprocket that has a great difference in teeth compared to the previous one. Therefore, it is possible that the cyclist is not able to find the optimum combination of sprockets and guide toothed wheels even with such a large number of sprockets.
In addition to the above, the total bulk of the sprocket assembly is very high at the bicycle wheel, for which reason the chain, when engaging the end sprockets, is inclined with respect to the guide toothed wheels and therefore more subject to friction, wear and tension peaks.